Well, since you have a vibrator on your wrist, you could place one contact there as well, a small metal object could be the other contact, forming your switch.

When you detect a drastic change in either resistance or capacitance between the two contact, then you know the person has fallen asleep. Let me search for a few simple circuits. This is much easier than the sensitive electro-galvanic stuff we were doing earlier.

http://www.discovercircuits.com/S/s-capacitance.htm

Hmm, maybe easier would be to hold a mercury switch in you hand. When you relax, the hand falls over tripping the switch.

So many options, so little time....